It is known to design such an oil separator as a housing with a removable separation unit or filter cartridge inserted therein, this unit including one or more filters to be traversed by the flow of gas admixed with oil. The heavier oil particles do not reach the filter assembly but are separated ahead of the cartridge from the upwardly or horizontally flowing gas stream; the remainder of the oil coalesces along the downstream surfaces of the filter elements and accumulates at the bottom of the cartridge while the purified gas flow exists through an outlet port thereof. Because of the pressure differential (e.g. of 300 mm water column or more) that must be maintained in order to drive the gas glow through the filter, the oil accumulating inside the cartridge cannot be simply discharged into the housing for return to the compressor along with the initially separated oil. The expedient of connecting the sump of the cartridge with the oil pool of the housing by way of a constricted drain is unsatisfactory inasmuch as the constriction, if wide enough to give passage to the relatively viscous cold oil during the starting-up period, will subsequently allow a substantial quantity of compressed gas to return along with the oil from the interior of the cartridge to the compressor intake with resulting diminution of the efficiency of the system. Attempts to eliminate the need for such a throttled return through the generation of a positive driving force, designed to overcome the aforementioned pressure differential, have heretofore involved relatively complicated structures which make the device difficult to assemble and service.